Protective control circuits



March 10, 1953 M. E. BIVENS 2,631,267

PROTECTIVE CONTROL CIRCUITS Filed Aug. 17, 1950 5 Sheets$heet 1 F3 .IA.

50 Inventor:

Maurice E. B\vens,

His Attorn y.

March 10, 1 953 BlVENs 2,631,267

PROTECTIVE CONTROL CIRCUITS Filed Aug. 17 1950 3 Sheets-Sheet 2 F i3. i5. 5 I19 a I653 4'6 23 7 15/ I 75 a "225 V -w I l z v I 76 as 109 m Inventor: Maurice E. B'wens, y @d 52 21,)?

His Attorney.

March 10, 1953 Filed Aug. 17 1950 M. E. BIVENS 2,631,267

PROTECTIVE CONTROL CIRCUITS 3 Sheets-Sheet 5 Figlc.

Inventor: Mauxice E.Biver1s,

y flaw WM His Attorney.

Patented Mar. 10, 1953 PROTECTIVE CONTROL CIRCUITS Maurice E. Bivens, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application August 17, 1950, Serial No. 179,965

24 Claims.

My invention relates to protective control circuits and, more particularly, to protective control circuits for rectifier systems such as may be used to energize electrostatic precipitators.

Electrostatic precipitators have many applications, for example, removal of fly ash from fiue gases and removal of valuable material suspended in gases resulting from industrial processes or conversely providing clean air for use in industrial processes by removing contaminating particles therefrom. These precipitators may be of several types, but in each case have one or more discharge electrodes and one or more collecting electrodes. The gas to be treated is passed between these electrodes and the solid matter in this gas is deposited on the collecting electrode or electrodes from which it is periodically removed. This removal may be accomplished by a rapper mechanism which acts on the collecting electrode or electrodes to jar the deposit therefrom.

These precipitators are low current, high voltage, direct current devices and operate most efficiently at a voltage level which is just below the value at which arcing occurs between the electrodes. Consequently, some arrangement must be provided for preventing damage to the electrical equipment as well as to the precipitator whenever arc-overs occur in the precipitator. Various means may be provided for limiting this surge current, such as employing a power transformer with high leakage or by connecting a resister or inductance in the input or output circuit of the rectifier employed for supplying voltage to the precipitator. Such arrangements, however, are not as desirable as a constant voltage arrangement wherein no provision is made for obtaining the drooping voltage characteristic resulting from the arrangements above specified. However, when a constant voltage arrangement is employed, it is necessary to provide a control which will very promptly shut down the system in case of overloads such as may result from arc-overs in the precipitator.

Where a full Wave rectifier is employed for supplying direct current to the precipitator, it is also desirable, in the interests of economy, to operate the rectifier as near as possible to its full load capacity. With such an arrangement, however, partial failure of the rectifier resulting in half wave rectification will overload that part of the rectifier still functioning because theprecipitator is essentially a capacitance load andwith half wave rectification the greater leakage time thus provided results in a greater current demand 2 on that portion of the full wave rectifier which is supplying this half wave energization of the precipitator. It is, consequently, desirable to provide the precipitator control with means re sponsive to a skip in half cycles of rectifier output for shutting down the control when such partial operation of the full wave rectifier takes place.

It is also desirable to have a control which will always fail safe, so that if any component part thereof does fail, the system Will shut down and thus avoid any possibility of damage due to undesired operation or faulty operation of the system resulting therefrom.

It is an object of my invention to provide an improved control circuit which will promptlyinterrupt the fiow or" load currenthwhenever this load current is above or below a predetermined safe value.

It is also an object of my invention to provide an improved control circuit which will automatically re-initiate its operation a predetermined time after shut down resulting from overor under-current flow in the load circuit.

It is another object of my invention to provide an improved control circuit which will interrupt the supply of load current whenever the frequency at which overloads occur is greater than a pre-established rate.

It is also an object of my invention to provide a control circuit in which dependable operation is insured by the arrangement of its component parts.

Further objects of my invention will become apparent from the embodiment thereof illustrated in Figs. 1A, 1B, and 10 which, when placed side by side in the order stated, diagrammatically represent my invention as applied to the control of a full wave rectifier having its input terminals connected to a source of alternating current supply and its output terminals connected to the electrodes of an electrostatic precipitator.

In accordance with the illustrated embodiment of my invention, current is supplied through load conductors to the electrodes of an electrostatic precipitator by a full wave rectifier whose connection with alternating current supply conductors is determined by a circuit controlling means. This circuit controlling means is operated to supply current to the rectifier by the alternate conduction in leadin and trailing fashion of a pair of electric discharge devices which, for convenience, will be identified as load current flow controlling electric discharge devices. If either of these electric discharge devices of this pair fail to conduct, the circuit controlling means is operated to prevent the flow of work current through the rectifier to the precipitator.

Conduction of this pair of electric discharge devices is initiated by conduction of an electric discharge device which is rendered non-conductive for a predetermined time interval depending on conduction .of the trailing load current flow controlling electric discharge device of this pair which is initially rendered conducting thereby and thereafter rendered conducting by concluction of the leading electric discharge device of this pair. Thus, if current flow through the load current flow controlling electric discharge devices is interrupted, this current initiating electric discharge device cannot re-initiate conduction of this pair until after the expiration of a predetermined time delay.

Continued operation of this pair of load current flow controlling electric discharge devices is made dependent upon .the sequential conduction in trailingiash'ion.ofanother pair of control electric discharge,deviceseach of whose conductivity is madedependent upon the flow to the precipitator ofrectifiedcurrent greater than .a predetermined minimum. Thus, the conductivity of the trailingcontrol-electric discharge device is dependent .on..,prior.conductivity of the leading control electric. discharge device and conductivity of both is dependent on thefflow of consecutive positive and negativehal'f .cycles of rectified current j'fiow in the 'load circuit. If the trailing electric discharge device of this control pair fails to conduct, .theleadingelectric discharge device of the currentlflow controlling pair of electric discharge devices .will not conduct and the circuit con'trolling means will be operated to prevent the flow of work current to the precipitator until reoper- .atedby their renewed conduction after the time delay imposed by the current initiating electric :discharge device. A skip of a half cycle ofrecti- Lfiedcurrent flow in thelload circuit of the rectifier will consequently operate through these control electric discharge devicestointerrupt conduction through the pair of load-current flow controlling electric discharge devices.

Thecontinued flowof current through the pair of load current flow controlling electric discharge .devices is also vinterrupted whenever an overload electric discharge device becomes conducting and {applies ahold-ofi voltage to the-leading electric discharge devices of this pair. This overload elec- ;,tric discharge device is prevented from operating:fonapredetermined time interval each time zthe' systemis set ,in operation so as to prevent .overloadresponse during the period .of timere- :guired for charging the precipitator and its cir- -cuit which constitutes a capacitance load and, ,as such, .will draw overload current while being :charged each time the system is set in operation.

.If the frequencyof overload current interruptions is greater than a predetermined rate, the :repeatcd operation of the overload electric discharge device will alter the conductivity of an- -otherelectric discharge device which is responsive .to this frequency of overload. The alteration in conductivity of this frequency of overload electric discharge device will initially operate a rap- .per which acts to clear deposits from the col- .lecting electrode of the precipitator and, after a',.predetermined time delay, will shut down the system if the frequency of overload does not .clecrease to below the predetermined rate.

.Each'time there is a skip in the rectified half "cycles of "the load current flow, another electric discharge device operates 'to shut the system down. This shut down electric discharge device has its conductivity altered in accordance with the failure of the trailing one of the pair of control electric discharge devices to operate whenever such skip occurs, thereby permitting the trailing one of the pair of load current flow controlling electric discharge devices by "its conduction to operate on the shut down electric discharge device to alter its conductivity.

.A detailed description of my control will now be given with reference to the diagrammatic representation-of the embodiment illustrated in Figs. 1A, 1B andlC of the drawings.

,As shown in the upper Fig. 1C portion of these drawings, the discharge electrode l and the collecting electrode 2 of an electrostatic precipitator t are'respectively connected to the output terminals of a power rectifier "i. The collecting electrode 2 of this precipitator is connected to ground at 5 and a potentiometer 5 is connected in that one of the load circuit conductors extending from one of the output terminal of the rectiher to the collecting electrode 2 of the precipitator.

The input terminals of the rectifier 4 are respectively connected to the secondary terminals of a constant high voltagepower transformer l whose primary is connected to supply conductors 3 through aplurality of circuitcontrolling means .one of which is "a'magn-etic contactor9 and the other of which is a controllable impedance In. When the contacts 1 i of the magnetic contactor' 9 are closed, the voltage applied to the primary'o'f the power transformer l dependson the impedance value of thecontrollableimpedance Ill.

As shown, this controllable impedance is 'a series transformer l2 having its primary 13 series connected between the supply conductorsfi and the primary of the power transformer I. The secondary M of this series transformer 12 is provided with end terminals which are connected through electric discharge devices 15 and It to a mid tap thereof which as illustrated may be grounded. Whenever the electric discharge devices l5 and It are conducting the secondary of theseries transformer 12 is shorted and this reduces the impedance of its primary winding to a low value so thatsubstantially the full'voltage of the supply conductors is applied to the terminals of the primary winding of the power transformer I. When the electric'discharge devices 15 and i5 arenonconducting, the secondary N of Ttheseries controlling transformer is open circuited and the impedance of. its "primary winding it is a maximum andabsorbspractically the full voltage of the supply conductors i8'so as effectively to deenergize the power transformer 'Pan'd the rectifier 4 fed therethrough which of course deenergizes the ,precipitator 3 by reducing to --a very low value or zero the work current'supplied thereto.

Each of the electric discharge devices l5-and I6 is provided with an anode I1, a cathode l8 and a plurality of control elements l9 and 20, all of which are enclosed within an envelope which may contain an ionizable gas or vapor as is indicated by the dot associated with their cathodes. The short circuiting connection for the secondary M of the series transformer 12 above describedis completed through the anode-cathode circuits of these electric discharge devices. Protection is provided against faulty operation of the electric discharge devices due to anode voltage surges by capacitors 2| connected between control elements 19 and cathodes l8 .of

these electric discharge devices. Spark gaps 22 likewise connected are also provided for protection against over-voltage between control ele ment 9 and cathode l8 due to anode voltage surges.

Control elements 29 ofelectric discharge devices I5 and7|6 are directly connected to their cathodes l8 and their control elements I9 are respectively connected incontrol element circuits including a conductor 23 and a .current limiting resistor 24 and a conductor 25. and a current limiting resistor 26. This control circuit also includes a conductor 21 which is connected to the cathodes l8 of these electric discharge devices.

.,'Ihe conductivity of electric discharge devices l5 and I6 is directly controlledibya pairof .electric discharge devices 28 and 29, a second pair of electric discharge devices 39 and 3| and electric discharge devices 32, 33 and 34 and indirectly controlled by the conductivities of electric discharge devices 35 and 36., Each of these electric dischargedevices is provided with an anode and a cathode and two control elements all of which are enclosed within an envelope preferably containing an ionizable gas or vapor as indicated by the dots associated with their cathodes. These anodes, cathodes. and control elements have been identified by the same reference numbers |1, |8, l9 and 29 employed for identifying like elements of electric discharge devices It? and i6 above described. Also, as disclosed above in connection with electric discharge devices l5 and Hi, control elements 9 of each of the electric discharge devices28-36 are connected to their cathodes |8 by a transient suppressing capacitor 2|.

Electric discharge devices 2836 have already been identified above in the general description in accordance with their primary functions. Thus, the pair of electric discharge devices 28 and 29 have been identified as load current flow controlling electric discharge devices, the pair 30 and 3| as control electric discharge devices, 32 and 33 as overload electric discharge devices, 34 as a current initiating electric discharge device, 35 as a shut-down electric discharge device, and 36 as a frequency of overload electric discharge device.

Anode-cathode voltage is supplied to the electric discharge device 2836 by a transformer 31 having a mid tap secondary winding 38 and a primary winding 39. Each of the cathodes l8 of electric discharge devices 2836 is directly connected to a conductor 40 which is connected to the mid tap terminal of the secondary winding 38 of transformer 31. The anodes of these electric discharge devices are connected to conductors 4| and 42 through circuits which will later on be described in detail. Conductor 4| is connected to the upper end terminal of the secondary 38 of transformer 31 through a conductor 43, contacts 44 of relay 45 and a conductor 46. Conductor 42 is connected with the lower end terminal of secondary 38 of transformer 31 through a conductor 41, contacts 48 of relay 45 and a conductor 49. Thus, when conductor 4| is at positive polarity relative to conductor 49, conductor 42 will be at negative polarity and vice versa. devices 3|, 29, 32, 33 and 36 are connected for conduction during one polarity of the supply voltage and electric discharge devices 33, 28, 34 and 35 are connected for conduction during the other polarity of this supply voltage. This volt- Consequentially, electric discharge age is derived from the supply conductors 8 by connecting the upper terminal of the secondary 39 of transformer 31 through conductors 50 and 5| to one of the terminals of the secondary 52 of a control circuit transformer 53 whose primary 54 is connected across the supply con-' ductorsB and by connecting the lower terminal of primary 39 of transformer 31 through a conductor 55, contacts 56 of a relay 51 and con ductors 58 and 59 to the other terminal of the secondary 52 of transformer 53. The anode-cathode circuits of electric discharge devices 30, 3|, 28, 29, 32, 33 and 34 are completed, through inductive circuits so that once these discharge devices have been rendered conducting duringpositive half cycle of their anode voltage, current flow continues over into the next negative half cycle producing a voltage drop thereacross from anode to cathode of these devices which is used as a control voltage for other electric dischargedevices whose anode voltage is positive during this next half cycle. This will become apparent as the descriptionproceeds.

Theanodes of electric discharge devices 28 and 34 are connected together by conductors 69 and tlandthrough conductor 60, the primary 6| of a control transformer 62, conductor 63, contacts 64 of a relay 65, conductor-'66, contacts 61 of a relay 63, conductor 69, contacts 19 of a relay 1| and conductor 12 to'conductor 42 previously described as energized by the secondary 38 of transformer .31. The anode of electric discharge device 29 is connected through conductor 13, the primary 14 of a control transformer 15 and conduc tor 16 to conductor 4| also energized by the sec ondary 38 of transformer 31 as above described. A control voltage is applied in the control ele-' ment circuit of control element 20 of electric discharge 34 by connecting it through a current limiting resistor 11 to terminal 18 of a phase shift circuit comprising a capacitor 19 and a resistor which are connected in series with one another across conductors 4| and 42. The arrangement is such that the electric discharge device 34 is released for conduction by its control element 29 only during the early part of a half cycle of its positive anode voltage. The other control element |9 of electric discharge device 34 is con nected with its cathode through a current limiting resistor 8|, a timing circuit including a ca pacitor 82 and its parallel connected adjustable resistance 93 and fixed resistance84, conductor 85.,a secondary 86 of a controltransformer 81, conductor 88, a secondary 89 of control transformer 15 and conductor 90 to conductor 4,0which is, as previously described, directly connected to the cathode of this electric dischargeidevice. The primary 9| of control transformer .81 and its parallel connected resistor 92 is connected through the anode-cathode circuits of electric discharge devices 32 and 33 across conductors 40 and 4|. Consequently, when either of the electric discharge devices 32 and 33 or electric discharge'de'vice 29 conduct, they energize the'pri- 'mary 9| of control transformer 81' or the primary oif voltage to control element IQ of electric dis;

charge device 34 which cannot again conduct for a predetermined time interval thereafter which iS-'?dcte1'1'ninfi3by -the:set g i sdjustable re istsince-18.3 connected Few-cross this capacitor 92.

anode-cathode.condu eleciricidischarge device 34. causes electric discharge device 29 to conductin trailingresponse during .thernext positive half -:cy.cle;of the-supply voltage by reason-of the-voltage drop from-anodeto cathode of electric discharge device 34 continuing I over into the next half cycle-of voltage when theanode voltage of electric discharge device 29 is positive. This control voltage'is applied in the-control element circuitlof control element I9 of electricvdischarge device 29 through lcurrent limiting re sistor-93 and-the negative self biasin circuit .94 which is connected to the anode of electric :dischargerlevi ce 34. The other control element .29 of-zelectric discharge device 2.9.is connected directly'toiconductor 49 which is connected to the cathode of thiselectric discharge device. At the time -the system is set in operation a negative biasvoltage is applied in the control element circuit of control element I9 ofelectric discharge device 29 through resistor 93, negative self biasi-ng. circuit 94, conductor v69,'secondary 6| .of .control transformer 62 and resistor 95 which is connected-through. conductor I2 to supply conductor -42. Resistance 95 is of-a sufiiciently high value .topreventionization of electric discharge devices 2813!}(1734 when-supplygconductor 42 is positive and thus applies positive anode voltage to these electric discharge devices.

Thecontrol element 29 of electric discharge device'28 is connected through a current limiting resistor 96 to the anode of electric discharge device 29. Asr'previously. stated, the anode-cathode circuit of electric-discharge. device 29 is connected inaninductivecircuit provided; by the primary I4 of control transformer .15 -so that 4 anode-cathode conduction-ofthis electric discharge device con- :tinues over intothe negative half cycle and will apply arcontrolvoltage to the control element 29 of electric discharge device 28 which will render it conducting provided it is released for -conduction=by the-control voltage applied to its control element I9. This control element voltage is furnished by the anode-cathode drop of elec- .tric dischargedevice 3I through a circuitinclud- .ing'a current limiting resistor 91, a negative self biasing circuit 98 and the secondary!!!) of the .control transformer 81. This secondary 99 of control transformer 8! introduces a negative hold offvoltage into this controlelement circuitwhenever electric dischargedevices 32vand 33 conduct therebygenergizing the primary. windi-ng9 I of this control-transformer 81.

. Anode-cathodeconduction of either one of the electric I discharge devices 29 or 34, connect the primary Winding 9| of control transformer 52 across supply conductors 49 and 42 to energize this transformer and induces in its secondary I99 a voltage which appears across the resistor I9I in "the control element circuit of the control element I"9 of-el'ectric discharge device I5. The voltage across resistor I9I is =sufiicient to overcome the'bias voltage of capacitor I92 also connected in 'thisrcontrol element circuit a'nd thereby render electricdischarge device I conducting. The control element circuit of electric discharge device I5 extends from its control element [9 through resistor 24, conductor 23, resistor I9I,

conductor I93, capacitor I92 andconductor 21 back to the cathode I8 of this electric discharge device I5.

In,1il e manner, anode-cathode conduction of ele tricrdischar esd ic fl cnnfictsthe prim ry I4 of control transformer 1.5 acrosssllpply con ductors 49 and thereby energizing it and inJ-.-1

ducing in its secondary winding I94 a control voltage whichappears acrossresistor I which is connected in the control element circuit of the controlelement I9 of electric discharge device I91 The voltage across resistor I95 isa turn on voltage suflicient to overcome the biasvoltage of capacitor I92 also connected in this control element circuit. The control circuitfor control element I9 .of electric discharge device I6 extends from its control I9 through resistor 25, conductor 25, resistor I95, conductor I93, capacitor I92 and conductor 21 to the cathode I8 of this electric discharge device I9. Capacitor I92 above referred to is charged from the supply conductors 49, M and 42 through a full wave rectifier, each'branch of whichincludes a contact rectifier I98 -,and a current limiting resistor I99. Each of the control transformers 92 and 15 is provided with another secondary winding I95 and Nil respectively connected in series circuits with the secondary winding I94 and resistor I95 and secondary winding I99 and resistor I9I Thus, at the end of a half cycle during which the primaries 6i and E4 of these transformers were energized, the reversal of voltage occurring-during the next half cycle is employed as a turn on voltage in the control element circuits above traced for electric discharge devices I5 and I6.

Anode-cathode conduction of electric discharge devices 99 and 3| is dependent upon current flow greater than a predetermined minimum in the load circuit connectingthe precipitator 3 to the output terminals of the rectifier 4. This result is obtained by connecting in the control element circuits of control elements I9 of these electric discharge devices a voltage derived from the potentiometer 5 which is connected in the load circuit. The predetermined minimum value specified is determined by the adjustment of the slider one. potentiometer I I9 Which is connected through a fixed resistance III across a capacitor II2 whose top terminal is charged to a-positive polarity bycurrent supplied through rectifier I I3 and conductors H 2 which are connected to another secondary winding I N5 of transformer 31. The control element circuits of each of these electric discharge devices 39 and 3| extends from their control elements I9 through current limiting resistors II9 II'l, conductor IIB, the upper portion of potentiometer H9, conductor .l I9, the right-hand portion of potentiometer ,6,

and ground connection 5 toconductor 49 which is also grounded. vItinay prove desirable to provide a direct metallic connection between conductor 39 and theground terminal of potentiometer 5 in order, to insure the effectiveness of the control voltages introduced in these control element circuits by the potentiometersreferred to. It will be noted that the potentiometer 6 provides a positive turn on voltage which must be greater than the negative hold oif voltage of potentiometer Ill] before the electric discharge devices 39 and BI are released for conduction.

Anode-cathode conduction of electric discharge device EI also depends on the voltage applied to its control element 29. This control element is connected to the cathode of this electric discharge device through a circuit including the negative self biasing circuit I29 and the anode- ,cathodecircuitof electric discharge device-39.

d vi e 3 onductin i t an xthal cycleafter electric discharge device'39 was rendered con- I24 in series across the supply conductors 49 and .42.

Control element I9 of electric discharge device 39 is connected directly to its cathode and consequently conduction of this electric dis- .charge device depends solely on the voltage applied to its control element 29 by means of the circuit above described.

Anode-cathode conduction of electric discharge device 3I connects the primary I25 of con- :trol' transformer I 25 across the supply conductors 49 and 4|. This primary winding I25 is shunted by a resistance I21. When the primary winding I25 of transformer I29 is energized, a

voltage is induced in its secondary winding I28 which is sufficient to overcome the voltage induced into the secondary 89 of control transformer I5 when the anode-cathode circuit of electric discharge device 29 is completed at the same time. These windings are connected in series with one another in the control circuit of control element I9 of electric discharge device 35. This control element circuit for electric discharge .device 35 extends from its control element I9 through current limiting resistor I29, negative self biasing circuit I39, conductor I3 I, secondary winding I23 of control transformer I23, conductor 88, secondary winding 89 of control transformer I5 and conductor 99 to the cathode of this electric discharge device 35. Electric discharge device 35 has its other control element 29 connected through a current limiting resistance I32 toterminal I8 of the phase shift circuit I9, 89 whichas in the case of electric discharge device 1 34 only permits conduction of electric discharge device 35 to be initiated during the early part of a positive half cycle of its anode voltage. Thus, if the electric discharge device 3i fails to conduct at a, time when electric discharge device 29 does conduct the electric discharge device 35 will be rendered nonconducting. Electric discharge device 3I conducts in trailing response to conducticn of electric discharge device 39 and both of these electric discharge devices fail to conduct if the load conductor current flow is less than a predetermined minimum. Thus, a. skip in half cycles of rectifiedcurrent of the power rectifier 4' will cause electric discharge device 35 to become nonconducting. This skip may be caused by failure of one of the electric discharge devices in the Electric discharge device 35 became conducting when its anode-cathode vcircuit across supply conductors 49 and 42 was completed through conductors 66 and I33, contacts I34 of relay I35 and As will appear later when this connection is completed by closure of vcontacts I34 of relay I35 control transformers I5 and I26 are deenergized so that there is no control voltage in the circuit of control element I9 of this electric discharge device 35 and consequently it becomes conducting early in the cycle of its positive anode voltage as a, result of the control voltage applied to its control element 29.v

. including a resistor ISI.

This anode-cathode conduction of electric discharge device 35 energizes the operating winding I3I of relay 58 resulting in the closure of its contacts 57. To prevent chattering of this relay 68 a smoothing circuit consisting of a series connected resistor I38 and capacitor I33 is connected 'in the control element circuits of these control elements I9 the voltages of potentiometers I49 and 6. The control element circuits for control elements I9 of electric discharge devices 32 and 33 extend from the control elements through a current limiting resistor I4I, the upper portions of potentiometer I49, conductor H9, and the right-hand portionof potentiometer 5 to ground 5 and thence to conductor 49 which is also grounded and connected with the cathodes of these electric discharge devices. Potentiometer I49 is connected across capacitor II 2 and provides a negative hold off voltage which is overcome by the positive turn on voltage provided by potentiometer 6 when the load current flow is greater than a predetermined maximum.

As previously stated, each time the precipitator 3 is connected for energization, the charging current may be greater than the normal full load current supplied thereto. Thus, it is desirable to prevent the electric discharge devices 32' and 33 responding to the signal applied to their control elements I9 during this charging period. This is accomplished by connecting their control elements 29 through a current limiting resistor I42 and a timing circuit comprising a parallel connected adjustable resistor I43 and a capacitor I44, to the anode of electric discharge device 39. Thus, when electric discharge device 39 is nonconducting, the capacitor I44 of the timing circuit will be charged through the control element to cathode circuits of electric discharge devices 32 and 33 when supply conductor 42 is positive and this will apply a negative hold off voltage to these control elements 29 of electric discharge devices 32 and 33 for a predetermined time interval after electric discharge device 39 becomes conducting. The length of this time interval will depend on the adjustment of resistance I 43 which controls the discharge rate of capacitor I44.

Each time electric discharge devices 32 and33 become conducting, a voltage is induced in the secondary I45 of control transformerBI and this devices 32 and 33 which, of course, as previously stated depend on overload current flow in'the load conductors connected to the precipitator.

. The storage capacitor I4! applies a charge to a control capacitor I59 through a charging circuit Capacitor I4I also has a reset discharge circuit through resistor I 52 which is completed through the contacts I53 of a relay I54 when this relay is deenergized. Asa

result of the arrangement described, the charge on control capacitor I59 Will vary in accordance sistor I52. completes a the rapper and rapper control indicated at :w-ith-the frequency of operation of the electric .discharge devices 32 and 33 which are responsive .tocontrol element I9 of electric discharge device 36. Its other terminal is connected to a point (50 of a voltage divider formed by series connected resistors I55, I56, I51 and I58 which are connected across a capacitor I59. Capacitor I59 is: charged through a rectifier I60 connected in circuit therewith across supply conductors 40 and 42-.- Resistor I55 of this voltage divider isconnected in series with a resistor I6I across capacitor I02. Thus, the comon terminal of resistors I55 and I56 is more negative than conductor 40. With the arrangement provided, point I50" on this'voltage divider is positive relative to conductor 40 by an amount which results in anode conduction of electric discharge device 36 when thereis'no chargeon control capacitor I50. It is to be noted that the cathode of electric dis charge device 36 is connected to conductor 40 and its control element I9 is connected with this point I50" on the voltage divider through its current limiting resistor I50 and control capacitor I50.

When electric discharge device 36 becomes conducting,it connects the operating winding I54 of relay I54 across the supply conductors 40 and 4-I-. This operates the relay to open its contacts I53 and I62 and toclose its contacts I63. The opening of its contacts I53 disconnects the discharge circuit connected therethrough across storage capacitor I41 and closure of its contacts I63 shorts out resistor I51 which is one of the elementsin the voltage divider above described.

This'increases the positive bias on the control element I9 of electric discharge device 36 thereby insuring that relay I54 operates in a positive manner. If due to frequency of overloads the control capacitor I50 is sufficiently charged to produce a resultant negative voltage in the control element circuit of control element I9 of' electric discharge device 36, this electric discharge device will become nonconducting resulting in-the opening of its contacts I63 and the closing of itscontacts I53 and I62. The openings oititscontacts I63 reinsert the resistance I51 in the voltagedivider chain thereby applying a still more-negative voltage to the controlelement I9 of the electric discharge device to insure positiveoperation of relay I54. The closure of contacts I53, discharges storage capacitor I41 through its discharge circuits including re- The closure of its contacts I62 control circuit which energizes I64 through a circuit extending from one terminalof the secondary 52 of control circuit transformer 53 through a conductor I65, contacts I62 of relay I54, conductor I66, contacts I61 of a relay I68, and conductors I69, I and 5I to the other terminal of the secondary 52 of transformer; 5,3; It also completes a circuit through the operating winding I1I of a relay I68 through conductors I65, I66, push button contacts I12 and-conductors I13, 50 and 5I.

Positive operation of relay I54 is also insured by'impressing on control element 20 of electric discharge. device-36 a voltage which releases this electric discharge device for conduction only at the beginning. of a positive half cycle of its ap-- plied anode voltage. This result is obtained by connecting. its control, element 20 through its current limiting resistor I14 to point I15 of" the phase shift circuit comprising the resistor I16 connected in series'with capacitor I11 across the supply conductors M and 42. A smoothing circuit consisting of series connected capacitor I18 and resistor I19 is connected across the operating winding I54 of relay I54.

The control is readied for operation byclosing the filament on push button I80. This completes the energizing circuit for the" operating winding ISI of relay 51 through the filament of push button I82. The energizingcircuit for winding I8I of relay 51 extends from one, terminal of the secondary 52 of control circuit transformer 53 through conductor 59, push buttons I and I82, and conductors I83, 50 and 5I' to the other terminal of the secondary winding 52 of transformer 53.

When relay 51 picks up, it completes its holding circuit through its contacts I84 which are connected across the contacts ofpush button I80. It also closes its contacts I85 which completes a half voltage energizing circuit for the transformer I86 which supplies filament heating current to the arc discharge devices in the power rectifier 4. This circuit is completed from one terminal of the secondary 52 of control circuit transformer 53 through conductor 59, contacts I85 of relay 51, resistor I81, and conductor I 88 toone terminal of the secondary I89 of the filament transformer I86 whose other te'rminalis connected through conductor 5I to the" other terminal of the secondary 52 of" the control circuit transformer 53. The secondary I90 of transformer I86 is connected to each of the filaments of the arc discharge devices in the arms of the rectifier 4.

The closure of contacts 56 of relay 51 also connects the primary 39 of transformer 38 through a circuit previously described for energization by the secondary 52 of the control circuit transformer 53. When the transformer 31 is energized, a secondary winding thereof (not shown) supplies heating current to each of the heating filaments-associated with the cathodes of the electric discharge devices shown in the Figs. 1A and lB'sections of the-drawings. These heating filaments and their connections with this secondary winding'of transformer 31 havebeen omitted from the drawing in order to simplify it. The pickup of relay 51 also energizes a filament heating transformer for the filaments of electric discharge devices I5 and I6 by connecting this filament transformer across the secondary of the control circuit transformer 53. This transformer and its connections have likewisebeen omitted in order to simplify the drawing.

The pickup of relay 51 also closes its contacts I9I which prepare a circuit for energizing the operating winding I92 of relay I93. The pickup of relay 51 also closes its contacts I94 which connect the operating winding I95 of relay I96 for energization across the secondar'y'52 of control circuit transformer 53 through a circuit including conductors 59, 50 and 5I.

primary circuit of the filament transformer I86 to apply full voltage to the filaments of the rectifier e. The pickup of relay I45 also closes its contacts i5 and 58 which through circuits previously described connect the secondary 38 of transformer 31 to the supply conductors 4| and 42, also previously described.

When relay I35 picks up after its time delay in closing, it also completes a circuit through its contacts 253 which connect the operating windizig 255 of a relay 202 for energization from the secondary 52 of the control circuit transformer 53 through conductors 59, 53 and 5|. This relay 202 after a time delay period closes its contacts 203.

If the start push button 254 is now closed, it

will energize the operating winding I92 of relay I93 from one terminal of the secondary 52 of control circuit transformer 53 through conductor 59, start push button 204, contacts I9! of relay 51 which were closed by prior pickup of relay 51, a stop push button 205, normally closed contacts 206 of relay 1|, and conductors 55 and 5| to the other terminal of the secondary 52 of transformer '53. Pickup of relay I93 will close its contacts 201 to complete its holding circuit around the contacts of start push button 204. Upon pickup, this relay will also close its contacts 208 completing the circuit for the operating winding 209 of relay I35 from one terminal of the secondary 52 of control circuit transformer 53 through conductor 59, contacts 253 of relay 202 closed by prior operation of relay 252, normally closed contacts 2I0 of relay 55, and conductors 53 and 5| to the other terminal of the secondary 52 of transformer 53.

Pickup of relay I35 will close its contacts I34 completing the anode-cathode circuit previously 1 traced for electric discharge device 35. It will also close its contacts 2H which will complete the energizing circuit for the operating winding 2I2 of the line contactor 9 through a circuit extending from one terminal of the secondary 52 of control circuit transformer 53 through conductor 59, contacts 293 of relay 202, contacts 208 of relay I93, conductor 2I3, its own contacts 2| I, and conductor 5| to the other terminal of the secondary 52 of transformer 53.

When contactor 9 picks up, it closes its contacts l! thereby completing a circuit through controllable impedance I for energizing the primary of power transformer 1 from the supply conductors 8. The pickup of contactor 9 also closes its contacts 2 I4 to complete its holding circuit independently of the contacts 2 of relay I35. The pickup of contactor 9 also closes its contacts 2!? which completes the energizing circuit for winding 2I5 of relay 65 across the secondary 52 of control circuit transformer 53 through conductors 59, 53 and 5|. Pickup of relay 65 drops out relay I35 by opening its energizing winding at contacts 210. The pickup of relay 55 also closes its contacts 54 which completes the anode-cathode circuit of electric discharge devices 34 and 28 through the primary 6| of control transformer 52 in the manner previously described.

It has been previously described how dropout of relay I54 controlled by electric discharge device 35 completed the operating circuit of the rapper and its control I64 as well as the energizing circuit for winding I1I of relay I68. A redetermined time after this latter circuit has been completed, relay I68 picks up and opens its contacts I61 to deenergize the rapper and its control I64. Pickup of relay I68 also closes its 14 contacts 2I1 to connect the operating Winding 2 I8 of time delay relay 2 I9 to the circuit extending from one terminal of the secondary 52 of control circuit transformer 53 through conductor I55, contacts I62 of relay I54, and conductors I66, 50 and 5| to the other terminal of the secondary 52 of transformer 53.

After a predetermined time interval, time delay relay 2I9 closes its contacts 220 to energize the operating winding 22| of relay II from one terminal of the secondary 52 of control circuit transformer 53 through conductors 59, 50 and 5| to the other terminal of this secondary of the control circuit transformer 53. When relay 1| picks up, it opens its contacts ,10 which are in the anode-cathode circuit of electric discharge device 35 thereby deenergizing, this electric discharge device and the relay 68 whose winding I31 is connected in circuit therewith. Pickup of relay II also opens it contacts 206 which will deenergize relay I93, which will drop out opening the energizing circuit of contactor 9 which also drops out openingthe circuits through its several contacts. Pickup of relay II also closes its contacts 222 completing a holdingv circuit which may be opened by operating a release push button switch 223. 5

Provision is made for manual operation of the rapper and its control I64. Thisis accomplished by closing the contacts 224 of, a push button switch which completes the control circuit for the rapper from one terminal of the, secondary 52 of the control circuit transformer 53 through conductors 59 and 225, contacts I61 of relay I68, and conductors I69, I10 and 5| to the otherterminal of the secondary-52 of the transformer-.53. The push buttons controlling contacts I12 and 224 are interlocked so that closure of one opens the other, and vice versa.

The above description of ture of the control system illustrated in the drawings will now be amplified by again describing it with regard to its operation, although this of necessity involves some repetition. Before beginning this description of its operation, it should be noted that the system has been illustrated in its deenergized condition.

When the filament on push button, switch I is depressed it completes the energizing circuit for the operating winding |8| of relay 51 from one terminal of the secondary 52 of control circuit transformer 53 through conductor 59, the contacts of push button switch I80, the normally closed contacts of the filament ofi push button switch I82, and conductors I83, 50 and 5| to the other terminal of the secondary 52 of transformer 53. When this relay 51 picks up it closes its contacts I84 completing a holding circuit around the contacts of push button switch I80 which thereafter may be released. Pickup of this relay also completes a circuit through its contacts I85 which connects the primary I89 of filament transformer I86 across the terminals of the secondary 52 of control circuit transformer 53 through a resistor I81. This circuit extends from one terminal of the secondary 52 of transformer 53 through conductor 59, contacts I85 of relay 51, resistor I81, conductor I88, the primary I89 of transformer I86, and conductor 5| to the other terminal of secondary 52 of transformer 53. This applies half voltage to the filaments of the electric discharge devices in the power rectifier 4 which are connected to the output terminals of the secondary 90 of filament transform- E1 I I the physical struc- 17 19 and 80. Relay 68 will consequently pick up closing its contacts 61.

The pickup of relay I35 also closes its contacts 2 II to complete the energizing circuit of operating winding 2I2 of line contactor 9. This circuit is completed from one terminal of the secondary 52 of control circuit transformer 53 through conductor 59, contacts 293 of relay 292, contacts 298 of relay I93, conductor 2I3, contacts 2! I of relay I35, the operating winding 2I2 of contactor 9 and conductor 5i to the other terminal of the secondary 52 of transformer 53. This causes the con-= tactor 9 to close its contacts. The closure of contacts II of contactor 9 completes the connection of the primary of the power transformer I with supply conductors 8 through the controllable impedance I9. The closure of its contacts 2 I4 completes its holding circuit around contacts 2II of relay I35 and the closure of its contacts 2 I 5 completes the energizing circuit for the operating winding 2I6 of relay 65 from one terminal of the secondary 52 of the control circuit transformer 53 through conductors 59, 59 and 5| to the other terminal of this secondary of transformer 53. This will cause relay 65 to pick up.

Pickup of relay 65 causes it to open its contacts 2 I9 thereby deenergizing the operating winding 299 of relay I35 which will drop out opening its contacts. The opening of contacts 2II of relay I35 has no effect on the contactor 9 whose circuit is now completed through its holding contacts 2I4. The opening of contacts I34 of relay I35 does not deenergize relay 38 since upon pickup of this relay its holding circuit is completed through its own contacts 61, conductor 69, contacts 19 of relay II and conductor 12 to the supply conductor 42.

Pickup of relay 65 also closes its contact 64 thereby completing the anode-cathode circuits of electric discharge devices 28 and 34 from conductor 42 through conductor 12, contacts 19 and relay II, conductor 69, contacts 61 of relay 69, conductor 66, contacts 64 of relay 65, conductor 63, the primary winding 6I of control transformer 62 and conductors 69 and BI to the anodes of these electric discharge devices whose cathodes are directly connected to conductor 49. Electric discharge device 28 is not free to conduct since its conduction is dependent on the prior conduction, during the immediately preceding half cycle, of electric discharge device 29 whose anode-cathode drop must be applied to control element 29 of electric discharge device 23 at the same time that the anode-cathode drop of electric discharge device 3! is applied to con trol element I9 of electric discharge device 28.

Electric discharge device 34, however, is free to conduct at the beginning of a positive half cycle of its anode voltage as determined by the phase shift voltage from phase shift circuit 79, 99 which is applied to its control element 29. This result follows since it control element I9 is at cathode potential since its timing circuit capacitor 82 is not charged and the control transformers I5 and 81 are deenergized.

Conduction of electric discharge device 34 consequently energizes the primary SI of control transformer 62 whose secondary I99 will thus produce a voltage drop across resistor IIJI which is in the control element circuit of electric discharge device I5. This voltage drop across resistor I9I will overcome the negative bias voltage of capacitor I92 and thus cause electric discharge device I5 to conduct and short the secondary I4 of the series controlling transformer I 2. This will reduce to practically zero the impedance of the primary winding of this series controlling transformer and effectively connect the primary of the power transformer I to the supply conductors 8 which will result in energization of the power rectifier and the supply of a rectified half-cycle of current flow to the precipitator 3.

This first rectified half-cycle surge of current supplied to the precipitator 3 will produce a voltage in the potentiometer 6 which is connected in the load circuit and this voltage will overcome the negative bias of potentiometer H9 and apply a turn-on voltage to the control element I9 of electric discharge device 39. This will render this electric discharge device conducting and release electric discharge device 3I for conduction during the next half cycle provided a turn-on voltage is applied to the control element I9 of this electric discharge device 3|. Electric discharge device 3! conducts in trailing response to conduction of electric discharge device 39 because of the drop from anode to cathode of electric discharge device 39 carries over into the next half cycle due to the inductive nature of its anode-cathode circuit resulting from inductance I2I.

Conduction of electric discharge device 34 also results in conduction of electric discharge device 29 in trailing response thereto as a result of its anode-cathode drop carrying over into into the next half cycle when the anode voltage of electric discharge device 29 is positive. This anode-cathode voltage drop of electric discharge device 34 is applied through conductors BI and 99, negative elf-biasing circuit 94 and resistor 93 to control element I9 of electric discharge device 29. The other control element 29 of electric discharge device 29 is tied to its cathode so that electric discharge device 29 conducts in trailing response to conduction of electric discharge device 34.

Conduction of electric discharge device 29 energizes the primary 14 of control transformer I5 whose secondary I94 produces a voltage drop across resistor I95 which overcomes the bias of capacitor I92 and renders electric discharge device IG conductive. The voltage reversal in winding I96 of transformer 62 resulting from the deenergization of primary BI also assists. Conduction of electric discharge device It short circuits the secondary I4 of the series controlling transformer I2 during the next half cycle and thereby reduces the impedance of its primary winding I3 so that the power transformer I is energized during the next half cycle of the voltage cf the supply conductors !8. This causes a consecutive half cycle surge of current to flow through the rectifier to the precipitator 3.

This second consecutive surge of current supplied to the precipitator 3 produces a voltage in the potentiometer 6 connected in the load circuit of the precipitator which voltage overcomes the bias voltage from potentiometer H9 and applies a positive turn-on voltage to the control element I9 of the electric discharge device 3! which has already been freed for conduction by the prior conduction of electric discharge device 39 applying a turn-on voltage to the control element 29 of electric discharge device 3|. By providing the phase shift circuit I23, I24 and connecting the anode of electric discharge device 39 through inductance I2I to the common terminal I22 of resistor I24 and I23, conduction of electric discharge device 39 is made to continue over to and preferably slightly beyond the 90 degree point of its next half cycle of voltage when the anode voltage of electric discharge device 3| is positive. This insures response of the electric discharge device 3| to the peak value of the current surge flowing in the load circuit of the precipitator.

When both electric discharge devices 3! and 29' conduct at the same time, the negative holdoff voltage of secondary 89 of control transformer I5 is opposed by the positive turn-on voltage of the secondary 528 of control transformer E26 so that conductivity of electric discharge device 35 is maintained and relay 68 continues to be energized, completing its circuit through its contacts 61. If, however, either electric discharge device 39 or 3! fails to conduct, the voltage of the secondary 85 of control transformer will render electric discharge device 35 nonconducting and thus shut the system down. This result follows if there is a skip in the half cycle rectified current supplied by the power rectifier 4. If this skip occurred during the first half cycle of operation of the rectifier, the electric discharge device 3! would fail to operate thereby preventing electric discharge device 3! from operating in trailing response even though a turn-on voltage were applied to its control element It as a result of the rectifier operating during the next half cycle. operate during this next half cycle, the electric discharge device 3| would not become conducting, even though prior conduction of electric discharge device 39 released it for conduction.

Conduction of electric discharge device 29 energizes the secondary 89 of the control transformer l5 and this charges the capacitor 82 located in the control element circuit of thecontrol element IQ of electric discharge device 34. This applies a negative hold oif voltage to this control element and prevents electric discharge device 34 from again conducting until after a predetermined time after 29 becomes nonconducting. Although conduction of electric discharge device 34 is thus blocked out, conduction of electric discharge device 28 follows in trailing response the conduction of electric discharge device 29, provided, of course, that the anodecathode drop of electric discharge device 29 is applied to control element of electric discharge device28 and the anode-cathode drop of electric discharge device Si is applied to the control element is of electric discharge device 28, so that the carryover of current flow in the inductive circuits of electric discharge devices 2% and 3! simultaneously apply to the control elements I9 and 20 of electric discharge device 28 turn-on voltages which are effective when the anode of electric discharge device 28 is positive.

If an overload occurs in the load circuit of the precipitator the voltage drop across potentiometer I5 connected therein becomes sufficient to overcome the negative bias of potentiometer I43 and render the electric discharge devices 32 and 33 conducting. This result cannot occur, however, each time initiation of conduction of the precipitator occurs when the charging current may be greater than the overload current because of the time delay imposed by capacitor I44 connected in'the control element circuitsof the control'elements 20 of electric discharge devices 32 and 33. As was stated above, this capacitor I 44 applies a negative hold-01f voltage for predetermined time interval beginning with each starting'of the supply of current to the precipi- Of course, if the rectifier failed to tatori; After this time interval has elapsed, how-' ever, the anode-cathode drop of electricdischarge device 38 applies a turn-on voltage to the control elements 28 of electric'discharge devices-32 and 33 so that'their conduction is solely dependent on the control voltage applied: to their control elements I8.

If for some reason overload current fiowsin the load circuit as may result from arcin in the precipitator, the magnitude of current flow through the potentiometer 6 will produce a voltage drop therein suflicient to overcome the negative bias of potentiometer I40 and therefore cause electric discharge devices 32 and 33 to conduct as a result of the control voltage applied to their control elements I9. It is, of course, assumed that this overload is not due to the charging current supplied to the precipitator and its circuits and that consequently sufficient time has elapsed since the precipitator was set in operation so that the time delay of capacitor I44 is no longer effective and conduction of electric discharge device 30 has applied a control voltage to the control elements 20 of electric discharge devices 32 and 33 which released them for conduction.

Conduction of electric discharge device 32 and 33 connect the primary winding 9| of control transformer 87 across the supply conductors 4D and 4!. This induces a voltage in the secondary 99 of transformer 8'! which applies a holdoif voltage in the control element circuit of control element I9 of electric discharge device 28 which overcomes the anode-cathode voltage drop across the electric discharge device 3| and there'- fore, prevents electric discharge device 28 from conducting, This will prevent. conduction of electric discharge devices I5 and I 6 and result in the series transformer I2 interposing a high impedance between the supply conductors 8 and the primary of the power transformer 1, thus effectively limiting the work current to a very low value or zero.

Conoluctionv of electric discharge devices 32 and 33 in response to an overload will also induce in the secondary 86 of control transformer 8'! a voltage which will charge capacitor 82 in the control element circuit of electric discharge device so as to insure the proper time delay before each restarting of the system. This will occur irrespective of the operation of electric discharge device 29 also acting on the timing capacitor 82 in the same manner throughsecondary 89 of control transformer 75. A voltage will also be induced in the'secondary I45 of transformer 87 which will produce a chargingvoltage for the. storage capacitor M! to which the rectified current is supplied from winding I45 through rectifier I46.

If the frequency at which overloads occur is greater than a predetermined rate, the voltage of storage capacitor I 47 will increase and'exist for a suflicient length of time to charge the control capacitor I50 so that it presents a negative control voltage to the control element I9 of electric discharge device 36. This will cause this electric discharge device 36 to become nonconducting' and deenergize the operating winding I54 of relay I54. When this relay drops it will close its contacts I53 completing the discharge circuit across storage capacitor I4'I. It will also complete a circuit through its contact i63 which will short out resistor I5! in the voltage divider resistor chain so as to increase the negative voltage applied to the control element I9 of the elec- 21 tric discharge device 36, and thereby insure its nonconductivity and positive operation of relay I54.

The drop out of relay I54 also closes its contact I62 which completes the operating circuit for the rapper I64. This circuit extends from one terminal of the secondary 52 of the control circuit transformer 53 through conductor I65, contact I62 of relay I54, conductor I66, contact I61 of relay I68 and conductors I69, I and 5I to the other terminal of the secondary 52 of transformer 53. The completion of this circuit will operate the rapper to vibrate the collecting electrode 2 of the prec'ipitator 3 to dislodge therefrom any deposits that caused the arcing which led to overload conditions in the load circuit. At the same time that the rapper is energized the operating winding I 'II of relay I68 is also energized from one terminal. of the secondary 52 of control circuit transformer 53 through conductor I65, contacts I62 of relay I54, conductor I 66 and 225, contacts I12 of a push button switch, operating Winding In of relay I68 and conductors 50 and 5I to the other terminal of the secondary 52 of transformer 53. After a predetermined time delay, relay I68 will open its contacts I67, deenergizing the rapper and closing its contacts 2II to complete the energizing circuit of the operating winding 2I8 of relay 2I9. The energizing circuit for this relay is believed to be quite obvious in view of the previous description.

When relay 2 I9 picks up after a predetermined time delay it closes its contacts 220 to complete the energizing circuit for the operating winding 22! of relay II. This energizing circuit extends from one terminal of the secondary 52 of the control circuit transformer 53 through conductor 59, the contacts of reset switch 223, contacts 226 of relay 2I9, the operating winding 22I of relay II, and conductors 50 and 5I to the other terminal of secondary 52 of transformer 53.

Pickup of relay 'II will close its contact 222 completing its holding circuit about contacts 220 of relay 2I9. It will also open its contacts ID and drop out relay 68 whose energizing winding I3! is in series circuit with contacts I0 across supply conductors 40 and 42. Relay ll will also open its contact 266 thereby dropping out relay I93 the opening of whose contacts 208 will deenergize the operating winding 2I2 of the contactor 9. This contactor Will consequently open its contact II to disconnect the supply conductors from the power transformer I. After this has occurred the circuit may again be set in operation by opening the reset switch 223 and thereafter closing the start push button 264.

The circuit has been simplified by omitting therefrom certain signal lights and the operating circuits for these signal lights. Such signals are provided in order to warn the operator of failure in any respect in the system and the nature of this failure. The nature of the rapper and its control has not been illustrated and described since all that it is necessary to know about it is that it is set in operation under certain circumstances and when in operation acts to vibrate the collecting electrode of the precipitator to dislodge deposits therefrom.

It will be noted that the arrangement in the system is such that failure of any of the electric discharge devices results in shutdown of the system except for electric discharge devices 32 and 33 which have been paralleled so that failure of either one of these devices alone will not disrupt the desired operation of the system. It is quite 22 unlikely that both these electric discharge devices .32 and 33 will fail at the same time.

The system above described may be variously modified without departing from the spirit and scope of my invention. Thus, the controllable impedance I0 may be replaced by another circuit controlling means which is actuated by the electric discharge devices .28 and 29. For example, these discharge devices may be employed as the circuit controlling means in the power circuit or employed for controlling a pair of reversely connected electric discharge devices inserted directly in this circuit. It is also apparent that the several leading and trailing operations of the electric discharge devices above described may be accomplished in another manner than by using the voltage drop from anode to cathode of one electric discharge device carrying over into the next half cycle due to the inductive circuit in which the device is placed so as to control the conduction of another electric discharge device when its anode voltage is positive during such next half cycle. It is also, of course, apparent that the refinement resulting from providing control transformers 62 and 15 with pairs .of windings for introducing control voltages in the control element circuits of the electric discharge devices I5 and I6 is not necessary since a single secondary winding will be effective in providing an operative system.

In systems built embodying my invention the voltage of the supply conductors has been 440 volts and the output voltage of the power transformer has been 50,000 volts. It is, of course, apparent that other supply voltages and output voltages may be used without departing from my invention. Furthermore, in order to simplify the presentation of the precipitator the most simple form thereof has been illustrated. It will be apparent, however, that any type of precipitator may be used and it is to be understood that precipitators having a plurality of discharge electrodes and a plurality of collecting electrodes may be employed. These collecting electrodes may be in the form of plates between which the discharge electrodes are arranged.

It is also apparent that my invention is not limited in its application to electrostatic precipitators. It may be used with any type of translating device where protection is to-be provided due to overcurrent, undercurrent, or where the frequency of faults due to overcurrents are greater than a predetermined rate. Furthermore, one or more of the means responsive to overcurrent, undercurrent or frequency of overcurrent may be omitted Without departing from my invention when it is applied to certain types of translating apparatus.

Thus, while I have shown and described but one particular embodiment of my invention, and suggested certain modifications therein, it will be quite obvious to those skilled in the art that many other changes and modifications may be made without departing from my invention in its broader aspects, and I therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A rectifier system comprising a full wave rectifier having input and output terminals, load conductors connected to said output terminals of said rectifier, alternating current supply conductors, a circuit-controlling means connected between said supply conductors and the input terminals of said rectifier for controlling the supply of alternating current to said rectifier, and means including leading and trailing electric discharge devices each having an anode and a cathode reversely connected with said supply conductors and each having a control element connected with said load conductors and responsive to a skip in the half cycle rectified pulses of current flow in said load conductors for operating said circuit-controlling means after not more than a half cycle of the alternating current supply following said skip to prevent flow of work current through said rectifier.

2. A rectifier system comprising a full wave rectifier having input and output terminals, load conductors connected to said output terminals of said rectifier, alternating current supply conductors, a circuit-controlling means connected between said supply conductors and the input terminals of said rectifier for controlling the supply of alternating current to said rectifier, means connected with said load conductors and responsive to a skip in the half cycle rectified pulses of current fiow in said load conductors for operating said circuit-controlling means to prevent fiow of work current through said rectifier, and means for operating said circuit-controlling means to re-establish the flow of work current through said rectifier after a predetermined time delay initiated by the operation of said last-mentioned means.

3. A rectifier system comprising a full wave rectifier having input and output terminals, load conductors connected to said output terminals of said rectifier, alternating current supply conductors, a circuit-controlling means connected be tween said supply conductors and the input terminals of said rectifier, said circuit controlling means operating promptly to absorb and promp ly to re-establish substantially the full voltage of said supply conductors applied thereby to the input terminals of said rectifier and thus promptly prevent and promptly reestablish the flow of work current through said. rectifier, means connected with said load conductors and responsive to overload current flow in said load conductors for operating said circuit-controlling means to prevent promptly the flow of work current through said rectifier, and means for operating said circuit-controlling means after a predetermined time delay initiated by the operation of said last-mentioned means to re-establish promptly at substantially the full voltage of said supply conductors the flow of work current through said rectifier.

4. A rectifier system comprising a full wave rectifier having input and output terminals, load conductors connected to said output terminals of said rectifier, alternating current supply conductors, a circuit-controlling means connected between said supply conductors and the input terminals of said rectifier, said circuit controlling means operating promptly to absorb and promptly to re-establish substantially the full voltage of said supply conductors applied thereby to the input terminals of said rectifier and thus promptly prevent and promptly re-establish the fiow of work current through said rectifier, meansconnected with said load conductors and "responsive to a skip in the half cycle rectified'pulses of current flow in said load conductors for operatingsaid circuit-controlling means to prevent promptly the fiow of work current through said rectifier, means connected with said load con- 'ductors and responsive to overload current flow in said load conductors for operating said circuit-controlling means to prevent promptly the flow of work current through said rectifier, and means for operating said circuit-controlling means after a predetermined time delay initiated by the operation of either of said last two mentioned means to re-establish promptly at substantially the full voltage of said supply conductors the flow of work current through said rectifier.

5. A rectifier system comprising a full wave rectifier having input and output terminals, load conductors connected to said output terminals of said rectifier, alternating current supply conductors, a circuit-controlling means connected between said supply conductors and the input terminals of said rectifier, said circuit controlling means operating promptly to absorb and promptly to re-establish substantially the full voltage of said supply conductors applied thereby to the input terminals of said rectifier and thus promptly prevent and promptly re-establish the flow of work current through said rectifier, means connected with said load conductors and responsive to a skip in the half cycle rectified pulses of current flow in said load conductors for operating said circuit-controlling means to prevent promptly the flow of work current through said rectifier, means connected with said load conductors and responsive to overload current fiow in said load conductors for operating said circuit-controlling means to prevent promptly the fiow of work current through said rectifier, means for operating said circuit-controlling means after a predetermined time delay initiated by the operation of either of said last two mentioned means to reestablish promptly at substantially the full voltage of said supply conductors the fiow'of work current through said rectifier, and means connected with said load conductors and responsive to overload current faults of a frequency greater than a preestablished rate for interrupting the connection between said supply conductors and the input terminals of said rectifier.

6. A rectifier system comprising a full wave rectifier having input and output terminals, load conductors connected to said output terminals of said rectifier, alternating current supply conductors, a circuit-controlling means connected between said supply conductors and the input terminals of said rectifier, said circuit controlling means operating promptly to absorb and promptly to re-establish substantially the full voltage of said supply conductors applied thereby to the input terminals of said rectifier and thus promptly prevent and promptl r'e-establish the flow of work current through said rectifier, a relay system for controlling the connection of said supply conductors with the input terminals of said rectifier through said circuit controlling means and for rendering said circuit controlling means operative, means connected with said load conductors and responsive to a skip in the half cycle rectified pulses of current fiow in said load conductors for operating said circuit-controlling means to prevent promptly the flow of work current through said rectifier, means connected with said load conductors and responsive to overload current flow in said load conductors for operating said circuit-controlling means to prevent promptly the fiow of work current through said rectifier, means for operating said circuitcontrolling means after a predetermined time delay initiated by the operation of either of said last two mentioned means to re-establi'sh promptly at substantially the full voltage of said supply conductors the fiow of work current through said rectifier, means also responsive to a skip in the half cycle rectified pulses of current flow in said load conductors for rendering said circuit controlling means inoperative until again rendered operative by operation of said relay system to interrupt and thereafter re-establish the connection between said supply conductors and the input terminals of said rectifier, and means connected. with said load conductors and responsive to overload current faults .of a frequency greater than a preestablished rate for operating said relay system to interrupt the connection between said supply conductors and the input terminals of said rectifier. .7

7. Apparatus comprising alternating current supply conductors, load conductors; a full wave rectifier having input and output terminals, means connecting said load conductors with said output terminals of said rectifier, and meansfor controlling the flow of current from said supply conductors to said input terminals of said rectifier, said means including a pair of electricdischarge devices one of which has an anode, a cathode, two control elements and two control element circuits each of which connects a different one of its said control elements with its said cathode and the other of which hasan anodea cathode, acontrol element and a control element circuit which connects its said control element with its said cathode, means connecting the anode-cathode circuits of said electric discharge devices with said supply conductors so that said one of said electric discharge devices may conduct during half cycles of voltage of one polarity and the other of said electric discharge devices may conduct during immediately following half cyclesof the opposite polarity, means responsive 4 to anode-cathode conduction of said one of said electric discharge devices which was initiated during a half cycle of supply circuit voltage when its anode voltage was positive for introducing into said control element circuit of said other of said electric discharge devices a voltage which will initiate conduction of said other electric discharge device during the next half cycle oi said supply conductor voltage when its anode voltage is positive, and means for introducing into each of said control element circuits of said one of said electric discharge devices, voltages the simultaneous existence of which will initiate its conduction during half cycles. of said supply conductor voltage when its anode voltage is positive, the supply of one of said voltages in one of said controi'element circuits being dependent on anode-cathode conduction of said other of said electric. discharge devices which was initiated during an immediately preceding half cycle of supplyconductor voltage when its anode voltage was'positive and the supply of the other of said voltages in the other of said control element circuits being dc.- pendent on the flow of consecutive half cycle pulses of rectified current in said load conductors.

8. Apparatus comprising alternating current supply conductors, load conductors, a full wave rectifier having input and output terminals, means connecting said load conductors with said output terminals of said rectifier, and meansfor controlling the flow of current from supply conductors to said input terminals of said rectifier, said means including a current initiating-electric discharge ,device having ananode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, a pair of electric discharge devices one of which has an anode, a cathode, two control elements and two control element, circuits each of which connects a different one of its said control elements with its said cathode andthe other of which has an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, means connecting theanode-cathode cir. cuits of said electric discharge devices withsaid supply conductors so that said current, initiating electric discharge, device and said one of said pair of electric discharge devices may conduct during half cycles or voltage of one polarity and the other of saidpair of electriodischarge devices may conduct during immediately,following half cycles of the opposite polarity, meansre sponsive to anode-cathode conduction of either one of said current initiatingelectric, discharge device and said one of said pair of electric discharge devices which was initiatedduring a half cycle of supply circuit voltage when its anode voltage was positive for introducing into said control element circuit of said oth r of said pair of electric discharge devices a voltage which will initiate conduction of said other electric discharge deviceduring the nexthalf cycle of said supply conductor voltage when its anode voltage is positive, means for introducing into ,eachof said control element circuitsOf said one of said pair of electric discharge devices, voltagesthe simultaneous existence of which will initiat its c u i n during a icyc es of s u y conductor voltage when its anode voltage is positive, the supply of one of said voltages in one of said control element circuits being dependent on anode-cathode conduction of said other of said electric discharge devices which was initiated during an immediately preceding half cycle of supply conductor voltage when its anode voltage was positive and the supply of the other of said voltagesin the other of said control element circuits being dependent on the flow of consecutive half cycle pulses of rectified current in said l oad conductors, a timing capacitor and its parallel connected resistor connected in the control element circuit of said current initiating electric discharge device, means responsive to anodecathode conduction of said other of said pair of electric discharge devices for charging said capacitor through the control element to cathode circuit of said current initiating electric discharge device to apply a negative hold ofi Voltage in said circuit for a predetermined time interval after anode-cathode conduction of said other-of said electric discharge device stops, and means for controlling the anode-cathode connection of said current initiating electric discharge device and said one of said pair of electric discharge de vices with said supply conductors.

9. Apparatus comprising alternating current supply conductors, load conductors, a full Wave rectifier having input and output terminals, means connecting said load conductors with said output terminals of said rectifier, andmeans for controlling the flow of current front ,said supply conductors to said input terminals of said rectifier, said means including a first pair of electric discharge devices one of wh ch has an anode, a cathode, two control element and two control element circuits each of which connects a different one of its said control elements i h its, s d, a ho an the ot er of w i h hasan anode, a cathode, a control element and 27 a control element circuit which connects its said control element with its said cathode, a second pair of electric discharge devices one of which has an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode and the other of which has an anode, a cathode, two control elements, and two control element circuits each of which connects a diiierent one of its said control elements with its said cathode, means connecting the anode-cathode circuits of said pairs of electric discharge devices with said supply conductors so that said ones of said pairs of said electric discharge devices may conduct during half cycles of voltage of one pola ity and the others of said pairs of said electric discharge devices may conduct during immediately following half cycles of the opposite polarity, means responsive to anode-cathode conduction of said one of said first pair of electric discharge devices which was initiated during a half cycle of supply conductor voltage when its anode voltage was positive for introducing into said control element circuit of said other of said first pair of electric discharge devices a voltage which will initiate conduction of said other of said first pair of electric discharge device during the next half cycle of said supply conductor voltage when its anode voltage is positive, means for introducin into each of said control element circuits of said one of said first pair of electric discharge devices, voltages the simultaneous existence of which will initiate its conduction during half cycles of said supply conductor voltage when its anode voltage is positive, the supply of one of said voltages in one of said control element circuits being dependent on anode-cathode conduction of said other of said first pair of electric discharge devices which was initiated during an immediately preceding half cycle of supply conductor voltage when its anode voltage Was positive and the supply of the other of said voltages in the other of said control element circuits being dependent on anode-cathode conduction of said other of said second pair of electric discharg devices which was initiated during an immediately preceding half cycle of supply conductor voltage when its anode voltage was positive, means recontrol element circuit of said other of said second pair of electric discharge devices a voltage which releases it for conduction subject to the control voltage of its said other control element, and means responsive to rectified half cycle pulses of current flow in said load conductors which are greater than a predetermined minimum value for introducing a control voltage in the control element circuit of said one of said second pair of electric discharge devices which renders it conducting and in the other of said pair of control element circuits of said other of said second pair of electric discharge devices which renders it conducting subject to its release for conduction by the above mentioned voltage applied in its said one control element circuit.

10. Apparatus comprising alternating current supply conductors, load conductors, a full wave rectifier having input and output terminals, means connecting said load conductors with said output terminals of said rectifier, and means for controlling the flow of current from said supply conductors to said input terminals of said rectifier, said means including a current initiating electric discharge device having an anode, a cathode, a control element and a con,- trol element circuit which connects its said c trol element with its said cathode, a pair of load current flow controlling electric discharge devices one of which has an anode, a cathode, two control elements and two control element circuits each of which connects a different one of its said control elements with its said cathode and the other of. which has an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, a pair of control electric discharge devices one of which has an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode and the other of which has an anode, a cathode, two control elements, and two control element circuits each of which connects a different one of its said control elements with its said cathode, means connecting the anodecathode circuits of said electric discharge devices with said supply conductors so that said current initiating electric discharge device and said ones of said pairs of said electric discharge devices may conduct during half cycles of voltage of one polarity and the others of said pairs of said electric discharge devices may conduct during immediately following half cycles of the opposite polarity, means responsive to anodecathode conduction of either one of said current initiating electric discharge devices and said one of said pair of load current flow controlling electric discharge devices which was initiated during a half cycle of supply conductor voltage when its anode voltage was positive for introducing into said control element circuit of said other of said pair of load current flow controlling electric discharge devices a voltage which will initiate conduction thereof during the next half cycle of said supply conductor voltage when its anode voltage is positive, means for introducing into each of said control element circuits of said one of said pair of load current flow controlling electric discharge devices, voltages the simultaneous existence of which will initiate its conduction during half cycles of said supply conductor voltage when its anode voltage is positive, the supply of one of said voltages in one of said control element circuits being dependent on anodecathode conduction of said other of said pair of load current flow controlling electric discharge devices which was initiated during an immediately preceding half cycle of supply conductor voltage when its anode voltage was positive and the supply of th other of said voltages in the other of said control element circuits being dependent on anode-cathode conduction of said other of said pair of control electric discharge devices which was initiated during an immediately preceding half cycle of supply conductor voltage when its anode voltage was positive, means responsive to anodecathode conduction of said one of said pair of control electric discharge devices which was initiated during a half cycle of supply conductor voltage when its anode voltage was positive for introducing into said one control element circuit of said other of said pair of control elec- 29 tric discharge devices a voltage which releases it for conduction subject to the control voltage of its said other control element, means responsive to rectified half cycle pulses of current flow in said load conductors which are greater than a predetermined minimum value for introducing a control voltage in the control element circuit of said one of said pair of control electric discharge devices which renders it conducting and in the other of said pair of control element circuits of said other of said pair of control electric discharge devices which renders it conducting subject to its release for conduction by the voltage applied in its said one control element circuit, a timing capacitor and its parallel connected resistor connected in the control element circuits of said current initiating electric discharge device, means responsive to anode-cathode conduction of said other of said pair of load current flow controlling electric discharge devices for charging said capacitor through the control element to cathode circuit of said current initiating electric discharge device to apply a negative hold ofi voltagein said circuit for a predetermined time interval after anode-cathode conduction of said other of said pair of load current flow controlling electric dis-- charge device stops, and means for controlling the anode-cathode connection of said current initiating electric discharge device and said one of said pair of load currentflow controlling electric discharge devices with said supply conductors.

11. Apparatus comprising alternating current supply conductors, load conductors, a full wave rectifier having input and output terminals, means connecting said load conductors with said output terminals of said rectifier, and means for controlling the flow of current from said supply conductors to said input terminals of said rectifier, said means including a current initiating electric discharge device having an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, a pair of load currentfiow controlling electric discharged devices one. of which has an anode, a cathode, two control elements and two control element circuits each of which connects a different one of its said: control elements with its said cathode and theother of which has an anode, a cathode, a. control eleinent and a control element circuit which connects its said control element with its said cathode, a pair of control electric discharge devices one of which has an anode, a cathode, a control element and a control element. circuit which connects its said control element with its said cathode and the other of which has an anode, a cathode, two control elements, and two control element circuits each of which connects a different one of its said control elements with its said cathode, a shut down electric discharge device having an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, means connecting the anode-cathode circuits of said electric discharge devices with said supply conductors so that said current initiating electric discharge device, said ones of said pairs of said electric discharge devices, and said shut down electric discharge device may conduct' half cycles of voltage of one polarity and the others-of said pairs of said electric discharge devices may conduct during immediately followin ghalf cycles of the opposite polaritygmeans responsive to anode-cathode conduction of either one of said current initiating electric discharge devices and said one of said pair of load current flow controlling electric discharge devices which was initiated during a half cycle of supply conductor voltage when its anode voltage was positive for introducing into said control element circuit of said other of said pair of load current flow controlling electric discharge devices a volt age which will initiate conduction thereof dur: ing the next half cycle of said supply conductor voltage when its anode voltage is positive,'means for introducing into each of said control element circuits of said one of said pair of load current flow controlling electric discharge devices, voltages the simultaneous existence of which will initiate its conduction during half cycles of said supply conductor voltage when its anode voltage is positive, the supply of one of said voltages in one of said control element circuits being dependent on anode-cathode conduction of said other of said pair of load current flow controlling electric discharge devices which waS initiated during an immediately preceding half cycle of supply conductor voltage when itsanode voltage was positive and the supply of the other of said voltages in the other of said control element circuits being dependent on anodecathode conduction of said other of said pair' of control electric discharge devices which was ini tiated during an immediately preceding half cy-' cle of supply conductor voltage when its anode voltage was positive, means responsive to anodecathode conduction of said one of said pair of control electric discharge devices. which was initiated during a half cycle of supply conductor voltage when its anode voltage was positive for introducing into said one control element circuit of said other of said pair of control electric discharge devices a voltage which releases it for conduction during the next half cycle when its anode voltage is positive subject to the control voltage of its said other control element, means responsive to rectified half cycle pulses of current flow in said load conductors which are greater than a predetermined minimum value for introducing a control voltage in the control element circuit of said one of said pair of control electric discharge devices which renders it conducting and in the other of said pair of control clement circuits of said other of said pair of control electric discharge devices which renders it conducting subject toits release for con duction by the voltage applied in itscsaid one control element circuit, a timing capacitor and its parallel connected resistor connected in the control element circuitof said current initiating electric discharge device, means responsiveto anode-cathode conduction of said other of said pair of load current flow controlling electric discharge devices for charging said capacitor through the control element to cathode circuit of said current initiating electric discharge device'to apply a negative hold on" voltage-in said circuit fora predetermined: time interval after anode cathode conduction of said other of said pair of load current flow controlling electric discharge devices, and means responsive: to a change in the state of conductivity of said shut-'downel'ec tric discharge device for disconnecting the" anode cathodecircuits of said current" initiating electric discharge device and said one of said pair of load current flow controlling electric dis-'- charge devices. I

. 12.. Apparatus comprising alternating current am ss? 31 supply conductors, load conductors, a full wave rectifier having input and output terminals, means connecting said load conductors with said output terminals of said rectifier, and means for controlling the flow of current from said supply conductors to said input terminals of said rectifier, said means including a current initiating electric discharge device having an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, a pair of load current flow controlling electric discharge devices one of which has an anode, a cathode, two control elements and two control element circuits each of which connects a different one of its said control elements with its said cathode and the other or" which has an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, a pair of control electric discharge devices one of which has an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode and the other of which has an anode, a cathode, two control elements,'and two control element circuits each of which connects a different one of its said control elements with its said cathode, an overload responsive electric discharge device having an anode, a cathode, two control elements and two control element circuits each of which connects a different one of its said control elements with its said cathode, means connecting the anodecathode circuits of said electric discharge devices with said supply conductors so that said current initiating electric discharge device, and said ones of said pairs of said electric discharge devices may conduct during half cycles of voltage of one polarity and the others of said pairs of said electric discharge devices and said overload responsive electric discharge device may conduct during immediately following half cycles of the opposite polarity, means responsive to anode-cathode conduction of either one of said current initiating electric discharge devices and said one of said pair of load current flow controlling electric discharge devices which was initiated during ahalf cycle of supply conductor voltage when its anode voltage was positive for introducing into said con trol element circuit of said other of said pair of load current flow controlling electric discharge devices a voltage which will initiate conduction thereof during the next half cycle of said supply conductor voltage when its anode voltage is positive, m'eansfor introducing into each of said control element circuits of said one of said pair of load current fiow controlling electric discharge devices, voltages the simultaneous existence of which will initiate its conduction during half cycles of said supply conductor voltage when its anode voltage is positive, the supply of one of said voltages in one of said control element circuits being dependent on anode-cathode conduction of said other of said pair of load current flow controlling electric discharge device which was initiated during an immediately preceding half cycle of supply conductor voltage when its anode voltage was positive and the supply of the other of said voltages in the other of said control element circuits being dependent on anode-cathode conduction of said other of said pair of control electric dis-charge devices which was initiated during an immediately preceding half cycle of supply conductor voltage when its anode voltage was positive, means responsive to anode-cathode conduction of said one of said pair of control electric discharge devices which was initiated during a half cycle of supply conductor voltage when its anode voltage was positive for introducing into said one control element circuit of said other of said pair or" control electric discharge devices a voltage which releases it for conduction subject to the control voltage of its said other control element, means responsive to rectified half cycle pulses of current flow in said load conductors which are greater than a predetermined minimum value for introducing a control voltage in the control element circuit of said one of said pair of control electric discharge devices which renders it conducting and in the other of said pair of control element circuits of said other of said pair of control electric discharge devices which renders it conducting subject to its release for conduction by the voltage applied in its said one control element circuit, means responsive to rectified half cycle pulses of current flow in said load conductors which are greater than a predetermined maximum for introducing into one of said control element circuits of said overload responsive electric discharge device a voltage which renders it conducting subject to the restraint of its other control element circuit voltage, means responsive to anode-cathode conduction of said one of said pair of control electric discharge devices and initially efiective after a predetermined time delay for applying a voltage in the other of said control element circuits which in conjunction with said voltage in its said one control element circuit renders said overload responsive electric discharge device conducting, means responsive to anode-cathode conduction of said overload responsive electric discharge device for applying a hold oil voltage in said other of said control element circuits of said one of said pair of load current fiow controlling electric discharge devices, a timing capacitor and its parallel connected resistor connected in the control element circuit of said current initiating electric discharge device, means responsive to anode-cathode conduction of said other of said pair of load current flow controlling electric discharge devices and of said overload responsive electric discharge device for charging said capacitor through the control element to cathode circuit of said current initiating electric discharge device to apply a negative hold off voltage in said circuit for a predetermined time interval after anode-cathode conduction of said pair of load current flow controlling electric di'scharge devices and of said overload responsive electric dis-charge device stops, and means for controlling the anode-cathode connection of said current initiating electric discharge device and said one of said pair of load current fiow controlling electric discharge devices with said supply conductors.

13. Apparatus comprising alternating current supply conductors, load conductors, a full wave rectifier having input and output terminals, means connecting said load conductors with said output terminals of said rectifier, and means for controlling the flow of current from said supply conductors to said input terminals of said rectifier, said means including a current initiating electric discharge device having an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, a pair of load current flow controlling elec-- tric discharge devices one of which has an anode, a cathode, two control elements and two control gen e? 33 element circuits each of which connects a dif ferent one of its said control elements with its said cathode and the other of which has an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, a pair of control electric discharge devices one of which has an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode and the other of which has an anode, a cathode, two control elements, and two control element circuits each of which connects a different one of its said control elements with its said cathode, an overload responsive electric discharge device having an anode, a cathode, two control elements and two control element circuitse'ach of which connects a difierent one of its said control elements with its said cathode, a frequency of overload responsive electric discharge device having an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, a shut down electric discharge device having an anode, a cathode, a control element and a control element circuit which connects its said control element with its said cathode, means connecting the anode-cathode circuits of said electric discharge devices with said supply conductors so that said current initiating electric discharge device, said ones of said pairs of said electric discharge devices, and said shut down electric discharge device may conduct during half cycles of voltage of one polarity and the others of said pairs of said electric discharge devices, said overload responsive electric discharge device, and said frequency of overload responsive electric discharge device may conduct during immediately following half cycles of the opposite polarity, means responsive to anode-cathode conduction of either one of said current initiating electric discharge devices and said one of said pair of load current flow controlling electric discharge devices which was initiated duringa half cycle of supply conductor voltage when its anode voltage Was positive for introducing into said control element circuit ofsaid other of said pair of load current flow controlling electric discharge devices a voltage which will initiate conduction thereof during the ne'X't'hali cycle of said supply conductor voltage when its lano'de voltage is positive, means for introducing into each of said control element circuitso'f said one of said pair of load current flow controlling electric discharge devices, voltages the simultaneous existence of which will initiate its conduct-ion during half cycles of said supply conductor voltage when its anode voltage is positive, the's'up'ply of one of said voltages in one'of said control element circuits being dependent on anode-cathode conduction of said other of s'aid pair of load current flow controlling electric discharge devices which was initiated during an immediatel preceding half cycle of supply conductor voltage when its anode voltage was positive and the supply of the other of said voltages in the other of 'said control element circuits being dependent on anodeca'thode conduction ofsaid other of said pair of control electricdisch-arge devices which was ini-' control electric discharge devices which was initiated during a half cycle of-supply conduc tor voltage when its anode voltage was positive for introducing into said one control element circuit of said other of said pair of control electric discharge devices a voltage which releases it for conduction subject to the control voltage in its said other control element circuit, means responsive to rectified half cycle pulses of current flow said lead conductors which are greater than a predetermined mini-mum value for introducing a control voltage in the control element circuit of said one of said pair of control electric discharge devices which renders it conducting and in the other of said pair of control element circuits or saidother of said pair of control electric discharge devices which renders it conducting subject to its release for conduction by the voltage applied in its said one control element circuit, means responsive to half cycle pulses of current flow in said load conductors which are greater than a predetermined maximum for introducinginto one of said control element circuits of said overload responsive electric discharge device a voltage which renders it conducting subject to the restraint of its other control element circuit voltage, means responsive to anode-cathode conduction of said one of said pair of control electric discharge devices and initially effective after a predetermined time delay for applying in the other of said control element circuits or said overload responsive electric discharge device a voltage which in conjunction with said voltage in" its said one control element circuit renders said overload responsive device conducting,- means responsive to anode-cathode conduction of said overload responsive electric discharge device for applying a hold off voltage in said other of said control element circuits of said one of saidpair' of load current flow controlling electric discharge devices, a timing capacitor and itsparallel connect-ed resistor connected in the control element circuit of said current initiating electric discharge" device, means responsive to anode-cathode conduction of said other of said pair of load current flow controllingelectrlo dischargedevices and of said overload re'sponsive electric discharge device for charging said capacitor through the con-trol eleent tocathode oir-cuitof saidcurrent initiating electric discharge device toapply a negative hold off voltagein said circuit for a predeter-' mined time interval after anode-cathode conduction of said-other of'said pair of load current flow controlling electric discharge devices and of said overload responsive electric discharge device stops, bias voltage means connected in said control element circuitof said frequency of overload responsive electric discharge device for applying a voltage which controls its anodecathode conduc'tively, a controlvoltage'capacitor also connected in said control element circuit of said frequency of overload responsive electric discharge device, astor-age capacitor, means including a rectifier for charging said capacitor in response to anode-cathode conduction of said overload responsive electric discharge device, an adjustable discharge circuit connected across said storage capacitor, a charging circuit for said control voltage capacitor also connected across said storage capacitor for charging it to a polarity'which after a' time delay'r'everses the voltage in said control element circuit of said frequency of overload responsive electric discharge device from that'providedpy said'loias' voltage means,

means responsive: to conduction andnon-condum tron of said frequency of overload responsive 

